Parliament sat for the first time on a weekend in 37 years, fueling a dramatic showdown which saw the Commons approve an amendment which means legislation around the UK's divorce from the EU must be scrutinised and signed off before the country leaves the trade bloc.
So what happens next?
What is likely happen next to solve the Brexit standoff?
Before the European Union can ratify a deal, the House of Commons needs to do so.
Commons Speaker John Bercow rejected a Government bid to hold a meaningful vote on Boris Johnson's Brexit deal on Monday, saying it would be "repetitive and disorderly" and against the Commons rule book to do bring the same legislation before the House twice.
The government was aiming to pass key legislation this week, aiming for a October 31 Brexit, but MPs want to slow it down, giving time to properly scrutinise any new laws before they are passed.
Whilst the ball is still rolling on legislation, the deadline to get bills passed is tight - and the threat of a no-deal Brexit still looms at the end of the month.
The Government's focus has now switched bringing its WAB before MPs, with a vote on its second reading on Tuesday.
Ministers insist they "have the numbers" to push the agreement through, but the parliamentary situation appears to be on a tightrope.
Labour has made clear it will try to hijack the legislation by putting down amendments for a second Brexit referendum and a customs union with the EU.
Could Mr Johnson still get his deal through Parliament?
Yes, but time is running out before the October 31 deadline as the European Parliament would also need to ratify it, and it is unclear how soon MEPs will do that.
On Monday, the European Commission said the ratification process has been launched on the EU side.
But European Parliament’s chief Brexit official, Guy Verhofstadt, said last week MEPs will only start their work once the UK Parliament has passed a fully binding Brexit deal, and if that slips past the European plenary session this week, it may have to be picked up in the session that begins on November 13.
And without a meaningful vote in Parliament, support for the agreement has not yet been tested.
Though the PM has attracted support from a number of prominent Brexiteer Tories, including the European Research Group (ERG), the DUP is strongly opposed to the deal.
What is the WAB?
The WAB is the Government’s Brexit bill – the legislation needed for Brexit – which would implement the new deal agreed with the EU in UK law.
Brexit Secretary Stephen Barclay said: “If Parliament wants to respect the referendum, it must back the bill.”
What exactly happened on Saturday and why does it matter today?
His Brexit dream was scuppered, for now at least, by legislation put forward by Sir Oliver Letwin.
MPs voted by a majority of 16 to back the Letwin Amendment, which withholds approval of the Brexit deal agreed between Mr Johnson and Brussels "unless and until implementing legislation is passed".
One of the aims of this was to give MPs the time to properly assess legislation before it is passed.
Sir Oliver, who is one of the 21 MPs who lost the Tory whip for voting against the Government on Brexit, said the bill provides "insurance" against the UK departing the EU without a deal by default on the currently scheduled deadline of October 31.
What about the letters sent to the EU by Mr Johnson?
Under the terms of the Benn Act, which was passed against the PM’s wishes, the Prime Minister was compelled to write to the EU asking for a three-month Brexit extension if he had not secured a deal by 11pm UK time on October 19.
He told the Commons: “I will not negotiate a delay with the EU, and neither does the law compel me to do so.”
But the Prime Minister did send two letters to European Council president Donald Tusk.
First, there was an unsigned photocopy of the request he was obliged to send under the Benn Act, followed by a letter explaining why the Government did not actually want an extension.
So will the EU grant an extension?
Despite European Commission president Jean-Claude Juncker raising doubts over another Brexit delay, the decision needs to be taken by all 27 remaining EU states, not him.
On Saturday night, Mr Tusk said he would now start “consulting EU leaders on how to react”.
The EU could set a different length to an extension, either shorter or longer than the three-month one cited in the Benn Act.
The EU may also decide not to formally respond to such a letter from the PM until it sees if Mr Johnson can get the Withdrawal Agreement Bill through Parliament.
Despite all this, the PM is insisting that the UK will still quit the EU in 10 days.
Will there be an emergency EU summit?
If the PM gets the Withdrawal Agreement Bill through, there could be a special gathering of leaders on October 28.
If the deal needs more time at that stage to get through Parliament, leaders could agree to a short extension.